The invention relates to an air duct of a motor vehicle.
The invention relates more precisely to an air duct of the structuring type, which supports some of the forces to which the vehicle is subjected, the air duct acting in particular also as a crosspiece forming part of the structure of the vehicle and supporting various accessories.
The invention aims to provide, in a simple, reliable and inexpensive manner, an air duct having a high degree of mechanical strength and satisfactory sealing with respect to the air that has to circulate inside it.
As indicated in the introductory part of FR-A-2 796 577, it is known to produce a motor vehicle air duct extending in a direction of elongation and comprising two half-shells arranged opposite one another, such that each half-shell has:                a tubular portion extending in the direction of elongation of the duct, the tubular portions defining between them an air cavity extending in said direction of elongation,        two lateral flanks extending one on each side of the tubular portion and each having a contact surface in contact with one of the contact surfaces of the other half-shell, and        a continuous weld seam extends substantially in the direction of elongation of the duct between the contact surfaces of the half-shells which are in contact with one another.        
Although such a solution provides sealing between the two half-shells, it is not entirely satisfactory as regards mechanical strength in the case of impact. The applicant has noticed that the continuous weld seam has the advantage over a discontinuous weld (for example an ultrasonic weld, which is constituted by a succession of spots) of providing sealing between the half-shells but that it also has a weakness in terms of mechanical strength.
When an accident occurs, the air duct is subjected to multi-axial mechanical forces in different regions located in particular at the steering column and the airbag module. Those forces have a component which shears the weld seam and when one of the weld seams is broken in one region, the breakage is propagated rapidly in the direction of elongation of the duct, so that the duct opens laterally. The mechanical strength of the duct is then considerably reduced.
This is particularly true in the case of front impact where the front supports of the vehicle have a tendency to move towards one another and to cause the structuring air duct to be bent.
consequently, the solution indicated in the introductory part of FR-A-2 796 577 has not proved to be mechanically satisfactory.
In addition, JP-A-61 160316 discloses an air duct comprising two half-shells which are in contact with one another by way of lateral flanks and which are held together by foaming. In order to prevent the foam from entering the inside of the duct, baffles formed by ribs projecting relative to the contact surfaces are provided.
However, the various embodiments illustrated in JP-A-61 160316 are not entirely satisfactory, in particular as regards the assembly of the two half-shells by screwing, which is very time-consuming.
Moreover, the technique of foaming is relatively complex to implement, can be used economically only in certain special cases (duct near the outer surface of the dashboard) and does not, as such, provide a high degree of cohesion between the half-shells and consequently a strong resistance on the part of the duct towards the stresses to which the vehicle is subjected.